Reflecting telescope and its production process

ABSTRACT

A reflecting telescope is described, in which the surface of the primary mirror is in the form of a pitch circular surface of a hypothetical, rotationally symmetrical large mirror. The optical axis parallel to the incident optical path and the focal point of the primary mirror and the large mirror are consequently identical. The primary mirror or pitch circular surface are located alongside the optical axis, so that all the optical and mechanical aids to be arranged in the vicinity of the focal point can be located outside the light incidence area and the reflection field, without optical correcting means being required.

FIELD OF THE INVENTION

The invention relates to a reflecting telescope with a tube and aprimary mirror located therein, the focal point of the primary mirrorbeing located outside the tube, as well as to a process for theproduction thereof.

BACKGROUND OF THE INVENTION

Reflecting telescopes are known, in which the optical axis of theprimary mirror is parallel to the tube or the light incidence direction.The adaptive optics with the reflecting mirror are located in the focalpoint in the light incidence area, which leads to shading.

In addition, Wilhelm Herschel discloses a reflecting telescope, whoserotationally symmetrical primary mirror is sloped with respect to thelight incidence direction in such a way that the focal point is locatedoutside the tube, so that the adaptive optics can be positioned outsidethe incidence of light. However, as a result of the mirror slope thelight paths differ, which leads to astigmnatic image distortions. Thus,a correcting plate ,must be used for compensating the different lightpaths.

SUMMARY AND OBJECTS OF THE INVENTION

The problem of the present invention solves, is to provide a reflectingtelescope of the aforementioned type, in which the effective radiationincidence area is kept free from optical functional elements, such ase.g. the reflecting mirror with its mounting and the like, but the focalpoint can still be sufficiently specifically fixed to ensure that nocorrecting optics are required.

This problem is solved in that the surface of the primary mirror isshaped as a pitch circular surface of a hypothetical, rotationallysymmetrical large mirror, whose optical axis is positioned alongside thepitch circular surface and is parallel to the light incidence direction.

Thus, the invention is based on the idea that a hypothetical,rotationally symmetrical large mirror, which cannot be manufactured as aresult of its dimensions, has a focal point located in the optical axis,which is free from astigmatic distortions and the like. However, thisfocal point is not only decisive for the complete large mirror surface,but also for all partial surfaces. Thus, on restricting to a partialsurface of the large mirror displaced with respect to the optical axisand which can be obtained as a result of the proportions, then theassociated focal point must be located outside the light incidence tubewithout this leading to optical disadvantages.

Thus, for example, a circular partial surface according to the inventionpositioned alongside the optical axis has the same intensitydistribution of the incident radiation energy in the focal point as arotationally symmetrical, polished, circular mirror with the samediameter, whose focal length corresponds to that of the inventivepartial surface. This means that for an incidence bundle of beams it iscompletely unimportant whether it strikes a rotationally symmetrical,polished primary mirror, whose optical axis is identical to therotational axis of symmetry, or a partial surface located alongside theoptical axis, whose diffraction aperture considered in the lightradiation incidence direction is identical with the diffraction apertureof the rotationally symmetrical, polished mirror. All that is importantis the length of the light path to the focal point, so as to occur therein an in-phase manner. This condition is fulfilled with the partialsurface according to the invention. It is a surface cutout of the largemirror and therefore images in completely distortion-free manner.

The invention has the advantage that with respect to the adaptive opticsand the associated functional elements, no constructional limitationsexist, because they are located outside the light/radiation incidencearea and therefore neither cause shading, nor project into thereflection field. As the focus of the primary mirror is kept free orundergoes no restriction in its action, a completely lossless lightconcentration in the focus of the primary mirror is possible.

Fundamentally the primary mirror can be constructed as a monolith.However, it is particularly advantageous for its surface to be formedfrom a plurality of segments. This not only makes it possible to producelarger diameter primary mirrors, but according to the inventionparticularly short focal lengths can be obtained, because the necessaryconcave curvature of the individual segments can be kept relativelyshallow. The tube length can also be shortened with the short focallength.

As a result of this size reduction the infrastructure of the reflectingtelescope can be made more economic and e.g. the protective donerequired for protection against the weather and external influences canbe made smaller.

It is particularly advantageous that a secondary reflecting mirror islocated in the reception area of the focus of the primary mirror and thesecondary mirror is movable at right angles to the optical axis of theprimary mirror. This has the advantage that without any longinterruption of telescope operation, it is possible to switch from oneobservation mode to the other. The secondary reflecting mirror can bemoved directly and in computer-controlled manner into the optical path,so as to permit the desired focus.

It is also advantageous that there is a second reflecting mirror movableparallel to the optical axis within the optical path of the firstsecondary reflecting mirror and which makes it possible to set differentfoci for telescope operation. This makes it possible to obtain veryshort switching operations between the focal areas, which can once againtake place automatically and in computer-controlled manner.

The mechanical means necessary for the switching operations, in the sameway as the secondary optical means and the in each case associatedcontrol means, can be given a particularly simple construction withrespect to their mounting, control, etc., in that all the observationpoints which can be controlled from the focal point are located in asingle plane passing through the optical axis and the centre of theprimary mirror. This has the advantage that the beam guidance and thetracking of the observation points can take place with especially simplemeans, particularly even in the case that the reflecting telescope isrotated about its horizontal mounting axis.

In the case of a telescope formed by mirror segments, the processproblem is solved in that individual mirror segment blanks are sopreshaped with a surface that in the case of continuous joining togetherof all the mirror segment blanks to form a primary mirror blank thesurfaces are initially summated to form a spherical cap, whose curvatureapproaches in optimum manner the aspherical final shape of the primarymirror and that subsequently the surfaces of the individual mirrorsegment blanks are reworked for producing the final surface.

This has the advantage that the primary mirror according to theinvention can be manufactured at limited cost, e.g. in a lightweighthoneycomb structure and mainly from quartz. The production costs duringpolishing are kept very low as a result of this preproduction processrelating to each mirror segment.

In the case of a diameter of approximately 5 to 8 m, the sphericalcurvature of the primary mirror blank diverges by approximately 1 mmfrom the aspherical final shape. Thus, only fractions of a millimetrehave to be polished in the reworking phase.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its uses,reference is made to the accompanying drawings and descriptive matter inwhich preferred embodiments of the invention are illustrated.

FIG. 1 Diagrammatically a plan view of a rotationally symmetrical,hypothetical large mirror with a pitch circular surface serving as theprimary mirror of a reflecting telescope.

FIG. 2 Diagrammatically a vertical cross-section through a reflectingtelescope.

FIG. 3 Diagrammatically and in vertical cross-section further details ofthe reflecting telescope according to FIG. 2.

FIG. 4 Diagrammatically a vertical cross-section through a primarymirror blank.

FIG. 5 Diagrammatically a vertical cross-section through a mirrorsegment blank and an associated blank body.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1 10 is a hypothetical large mirror 10, which in practice cannotbe produced in this size and which has a diameter B. The large mirror 10is aspherical and rotationally symmetrical, which is indicated by theconcentric circles 11. The hypothetical large mirror 10 is used forfixing the geometrical sizes of a pitch circular surface 12 with thediameter B. The diameter B of the pitch circular surface 12substantially corresponds to the primary mirror diameter of conventionalreflecting telescopes. In the represented embodiment the pitch circularsurface 12 is formed by means of a honeycomb mirror segment 13 to aprimary mirror 14 of a reflecting telescope 15 to be described ingreater detail relative to FIGS. 2 and 3. As illustrated in FIG. 1, thefocal point F of the hypothetical large mirror 10 is also the focalpoint of the pitch circular surface 12 or the primary mirror 14. Thus,with respect to their optical characteristics, the mirror segments 13are not rotationally symmetrical to the pitch circular surface 12 andinstead, for focussing on the focal point F, are rotationallysymmetrical to the large mirror 10.

The optical axis 16 passing through the focal point F is thereforesimultaneously the optical axis of the large mirror 10 and the pitchcircular surface 12 or the primary mirror 14. Focal point F is at apredetermined distance "a" outside the pitch circular surface 12, or, inother words, in the projection at right angles to the drawing plane,focal point F is outside the pitch circular surface 12 or the primarymirror. The distance of "a" is chosen in such a way that the adaptiveoptics 17 (FIG. 3) to be arranged in the focal point F and furtherfunctional elements necessary for deflecting the beams 18 (FIG. 2)reflected by the primary mirror 14 are positioned outside the pitchcircular surface 12, so that there is no shading of the light incidencebeans 19 (FIG. 2) at right angles to the drawing plane and thereflection field 20 (FIGS. 2, 3) is kept free.

In the cross-sectional view according to FIG. 2, the primary mirror 14or the pitch circular surface 12 is located in a tube 21 and is shown incontinuous line form. To facilitate understanding, the remainder of thehypothetical large mirror 10 is shown in broken line form. Tube 21 isparallel to the optical axis 16 and the light incidence beams 19. Acentral, incident light beam is designated 22 and the associatedreflection bean 23.

The reflecting telescope 30 according to FIG. 3 comprises a sphericalcasing 31, which is mounted in rotary manner by means of two tubular,walk-through, vertical shafts 32 in a not shown support structure, whichis e.g. described in DE-C-37 07 642. By means of the support structurethe spherical casing 31 can be pivoted both about the shafts 32 and withthe shafts 32 about a vertical axis 33. The latter is at the sane timethe longitudinal axis of tube 21, which is fixed in the spherical casing10. Primary mirror 14 according to FIG. 2 is mounted on the bottom oftube 21. Several observation cabins 34, 35, 36, 37 are located withinthe spherical casing 31. All the observation cabins 34 to 37 are locatedin a plane (drawing plane) passing through the optical axis 16 and thecentre of the primary mirror 14.

Between the access-providing, tubular shafts 32 and the observationcabins 34 to 37 are provided vertically directed elevators or lifts 38,39, 40, which are parallel to the optical axis 16.

In the vicinity of focal point F, there is movably located a firstconvex secondary reflecting mirror making it possible to modify thefocal length of primary mirror 14. The first secondary reflecting mirror41 reflects the beams focussed on focal point F on second convexsecondary reflecting mirrors 42, 43, 44, 45, which are movablypositioned parallel to optical axis 16 and in the plane of observationcabins 34 to 37.

FIG. 4 illustrates in broken line form and in cross-section aspherically curved primary mirror blank, which with a lightweighthoneycomb structure of individual mirror segment blanks 52 (FIG. 5) isformed into a spherical cap 51. The radius of the resulting sphericalcap 51 is R=2f, in which f is the vertical circle radius of the primarymirror 14 described relative to the preceding drawings. The concavesurface of the spherical cap 51 differs with minimum tolerances from theaspherical surface of the primary mirror 14, so that only slightpolishing is required.

In FIG. 5, 53 is a blank body with an aspherically curved surface, whichis used for producing a mirror segment blank 52. The latter can e.g. bemade from quartz or other equivalent, preshapable materials. Shapingtakes place under heat, pressure, etc. By juxtaposing a plurality ofsuch mirror segment blanks 52 a primary mirror blank in the form of aspherical cap 51 and as shown in FIG. 4 is obtained.

While specific embodiments of the invention have been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understand that the invention may be embodiedotherwise without departing from such principles.

I claim:
 1. A method of manufacturing a primary mirror for a reflectingtelescope with a tube and the primary mirror being located therein, thefocal point of the primary mirror being outside the tube, the surface ofthe primary mirror being aspherical and in the form of a circular pitchsurface of a hypothetical, rotationally symmetrical large mirror with anaspherical surface, whose optical axis is located alongside the circularpitch surface of the primary mirror and is parallel to the lightincidence direction, the method comprising the steps of:producing ofindividual mirror segment blanks, whose shape is determined by dividingthe aspherical surface of the circular primary reflector into singlesections; preshaping a mirror surface of the individual mirror blanks toform a spherical surface whose curvature approaches substantially theaspherical surface of the primary mirror; reworking of the mirrorsurface of the individual mirror blanks for producing a final asphericalsurface shape of the mirror surface of each mirror segment; and joiningtogether the reworked mirror blanks to form the primary reflector.
 2. Amethod according to claim 1, wherein the curvature of the sphericalsurface of the individual mirror blanks defers by approximately 1 mmfrom the final aspherical surface shape if a diameter of the primarymirror is approximately 5 to 8 m.
 3. A method according to claim 1,wherein: the mirror segment blanks are manufactured in a lightweighthoneycomb structure and predominantly from quartz.
 4. A method ofmanufacturing a primary mirror for a reflecting telescope with the tubeand the primary mirror being located therein, the focal point of theprimary mirror being outside the tube, the surface of the primary mirrorbeing aspherical and in the form of a circular pitch surface of ahypothetical, rotationally symmetrical large mirror with an asphericalsurface, whose optical axis is located alongside the circular pitchsurface or the primary mirror and is parallel to the light incidencedirection, the method comprising the steps of:producing individualmirror segment blanks having a light weight honey comb structure and amirror surface. preshaping a mirror surface of the individual mirrorblanks to form a spherical surface whose curvature approachessubstantially the aspherical surface of the primary mirror; reworking ofthe mirror surface of the individual mirror blanks for producing a finalaspherical surface shape of the mirror surface of each mirror segment;and joining together the reworked mirror blanks to form the primaryreflector.
 5. A method for forming a reflecting telescope, formed by thesteps of:designing an aspherical surface of a primary mirror to besubstantially similar to an aspherical surface of a portion of ahypothetical large primary reflecting mirror, said portion of saidhypothetical large primary reflecting mirror being spaced from anoptical axis of said hypothetical primary mirror; dividing saidaspherical surface of said primary mirror into a plurality of segments,each of said segments having an aspherical surface combining to formsaid aspherical surface of said primary mirror; creating a plurality ofmirror blanks with a mirror surface for each of said plurality ofsegments of said primary mirror; preshaping each of said plurality ofmirror blanks to form the mirror surface into a spherical shape whosecurvature is substantially similar to said aspherical surface of acorresponding segment of said primary mirror; reworking said sphericalsurface of each of said plurality of mirror blanks into said asphericalsurface of said corresponding segment of said primary mirror; joiningsaid plurality of mirror blanks together to form said primary mirror;positioning said primary mirror to receive incident radiation along adirection substantially parallel to said optical axis of saidhypothetical large primary mirror; and positioning a secondary mirror ata focal point of said hypothetical primary reflecting mirror and spacedfrom said incident radiation.
 6. A method in accordance with claim 5,wherein:said portion of said hypothetical primary reflecting mirror issubstantially circular.